Since electrical connections are both physically and functionally diverse, the background information given here applies in a general sense. When metal surfaces are pressed together, direct metal to metal contact is made at relatively few points, due to the roughness of the surfaces at the molecular level, even when the surfaces have been highly polished. At the molecular level, where electron transfer between two metal surfaces occurs as an electrical current is applied, a "clean" new connection is largely open space and non-conducting surface film. The load-bearing surfaces almost immediately acquire a film of occluded gases, moisture, oxides and other corrosion products before they can be brought together. The direct electron transfer which occurs in solid metal is hindered in electrical connections due to the roughness of the surfaces and the cushion of non-conducting film. In such new connections, these scattered points of electron transfer generally yield a contact resistance measured in millionths of an ohm. Although these values are low, the resistance is approximately 100 times that of a theoretically perfect connection, that is, one in which the contacting surfaces are completely bridged or replaced with solid metal. In general, the voltage drop across the low resistance of the new connection is too low to overcome the corrosive forces which immediately begin to attack the connection.
Electrical connections "breathe" due to expansion and contraction of the metal surfaces and of the gases and oxide films between the surfaces. As a result, as the connection alternates between a current-carrying state and a resting state, oxygen and other corrosive gases are pulled in and out, eroding away the edges of any points of solid metal contact. Even though the overall connection may seem cool, the actual points of current flow can become hot, even molten, due to their microscopic size and the relative absence of additional points of electron transfer. This leads to a high level of localized expansion, contraction, and chemical action which slowly destroys the initial point contacts within the connection. As this corrosion occurs, there is not yet sufficient voltage to initiate new paths through the non-conducting surface film by certain processes which have been identified and given such names as tunnelling, fritting, and micro-welding. This results in the scattered points of actual metal contact being corroded until the connection voltage rises, usually in six months or less, to approximately 0.1 volt where the micro-welding and other processes can maintain a balance at a given current.
At this point, if the connection has adequate heat dissipation and is not mechanically loose, this balance can be maintained. If 100 micro-welds are carrying a current of 10 amperes and the current is increased to 20 amperes, the voltage across the interface doubles to approximately 0.2 volt. This provides the energy to create 100 additional micro-welds and the voltage drops back to equilibrium at 0.1 volt. Either when or before this equilibrium point has been reached, several interacting conditions may come into play. The above-mentioned stabilization will be adequate for power connections if the heat generated by the increasing resistance (energy waste) is dissipated without undue increase in the overall connection temperature. However, since all of the connections in an enclosure are experiencing the same age-related rise in resistance, the entire unit becomes hotter with age. This is true particularly when some or all of the connections carry substantial power. Thus, a few of any substantial group of connections, due to a large number of interacting factors, may not reach stability but will instead accelerate toward physical and performance failure. An acceptable state of stability in a power connection may still be unacceptable in a sensitive connection, such as in a computer which carries some form of intelligence, or a connection in a shielding circuit where 0.1 volt may be a significant percentage of the signal voltage or of a self-generated shielding voltage. In these cases, "failure" may be a vague point at which the deterioration of performance over time, due to one or a number of connections, prevents satisfactory operations.
Previous efforts in this area have been mainly concerned with the control and prevention of corrosion and many formulations have been advanced for this purpose. However, with the aforementioned problems encountered in sensitive connections, corrosion prevention alone may be insufficient to prevent the deterioration of performance due to the relative absence of actual conductive paths which permit electron transfer. Recent studies have found that most failures in electronic performance are beyond the reach of present design and quality control. They can be traced instead to weak, misaligned, poorly assembled, or improperly serviced cable connectors, edge connectors, DIP and SIP sockects, relay sockets, shielding connections, and other mechanical connections which deteriorate randomly due to the factors discussed above.
It is important to clearly distinguish between the general class of formulations covered by this patent application and the large body of electrically conductive paints, inks, glues, and pastes which have been made since the 1940's. That the solution to these problems is not obvious may be seen from the fact that connection resistance and corrosion have been serious problems at least since the late 1800's, when the first Edison distribution systems and Bell telephone systems were installed. A very large number of attempts have been made to solve the problems. The corrosion aspect of the problem has been minimized in many ways. This new approach, to both reducing connection resistances by a substantial amount and to retaining the low resistances achieved, had not previously been discovered.
Only after extensive investigation and the development of a new body of theory, did the investigation lead to the present type of formulation, which lies outside of the formulating range of conventional conductive coatings. The superficial similarities between conventional conductive coatings and the subject matter of this application, have blocked rather than pointed to the development of materials to reduce and stabilize the resistance of electrical connections. The reason for this is that the conductive mechanism is quite different. The present, effective contact-enhancing materials use a ratio of the volume of conductive material to volume of carrier, which makes them electrically non-conducting, or insulators, when measured by the testing methods used in the conductive coating industry.
In conventional coatings, the volume of conductive material must be high enough relative to the volume of the carrier to provide dependable particle-to-particle conductive contact through the mass or film of the paint. This puts a lower limit of about 0.25 volumes of conductive material for each volume of carrier even for paints and inks in the megohm range. In contrast, the compositions which are the subject of this application function well with 0.15 volume or less of conductive material to one volume of carrier. The optimum ratio is usually about 0.05 volume of irregular or granular conductive material or about 0.1 volume of flake material to one volume of carrier.